The present invention relates to an air conditioning apparatus for a vehicle. More particularly, the present invention relates to an air conditioning apparatus using supercritical refrigerant, which is characterized by a refrigerating cycle to perform air conditioning while using the supercritical refrigerant.
Recently, the air conditioning apparatus for a vehicle is apt to employ, as a refrigerant in a refrigerating cycle, carbon dioxide gas as being a supercritical fluid maintained to be more than gas-liquid critical temperature and pressure. In such an air conditioning apparatus, even if the refrigerant leaks out, it is possible to reduce its influence on the environment due to the use of carbon dioxide gas as the refrigerant.
Generally, the refrigerating cycle includes a compressor, a condenser (or gas cooler), an expansion valve (throttle valve), an evaporator and an accumulator (low-pressure receiver). However, there is also provided a different refrigerating cycle to interpose an inside heat exchanger between the accumulator and the compressor. These air conditioning apparatuses are adapted so as to attain a cooling capacity with an optimum evaporating pressure on control of a quantity of circulating refrigerant and a pressure on the high-pressure side of the compressor by adjusting the opening degree of the expansion valve (see Japanese Patent No. 2931668).
In these air conditioning apparatuses each employing the supercritical refrigerants, there is an air conditioning apparatus that includes a compressor in the refrigerating cycle of carbon dioxide gas, the compressor being a variable displacement type compressor driven by an engine of a vehicle, and a control valve allowing a differential pressure between high pressure and low pressure in the refrigerating cycle to be adjusted constantly, the control valve being controlled by temperatures about an evaporator (e.g. temp. of refrigerant or blow-off temp). In such an apparatus, when the engine speed (number of revolutions) increases due to a vehicle's accelerating, the number of revolutions of the compressor is also increased simultaneously, so that a discharge amount of the refrigerant to be discharged from the compressor does increase. That is, an amount (referred “circulating refrigerant amount”) of refrigerant circulating in the refrigerating cycle is also increased.
Here noted, it is desirable that the apparatus is controlled so as to reduce the discharge volume of the compressor in order to make the above circulating refrigerant amount constant.
However, this restriction for discharge volume about the compressor causes a lower-side pressure in the refrigerating cycle and a refrigerant temperature to be reduced together. In such a case, the responsibility of the apparatus grows late due to the great influence of heat capacity of the refrigerant. While, since the apparatus is swift to respond against pressure fluctuation in the refrigerant, the control of the control valve takes a lead against a rising of a higher-side pressure in the refrigerating cycle.
Thus, since the control width of compressor displacement in reducing the flow rate of refrigerant is narrowed, there is a possibility that the circulation flow rate of refrigerant is increased out of control thereby increasing the cooling capacity of the air conditioning apparatus. Consequently, the engine output is excessively consumed in order to drive the compressor. Additionally, since both cooling capacity and compressor torque are excessively consumed during a period from the start of controlling the variable displacement in the compressor till the completion, there are induced various problems, for example, reduction in fuel consumption, reduction in accelerating capability, hunting in blow-off temperature, etc.